I feed mine crushed up flake food. They don't seem to eat it if you watch them, but if you leave them alone for like 2 hours with the food sprinkled on top, when you come back, most, or all of it is gone.
I put some of the flake food in a plastic bag, ziplock, and smash it up in there until it is almost powder. Then I just put a TINY pinch in the fry's tank.
That has worked for me so far.